The application of semiconductor chips or dice in the manufacture of light-emitting diodes (“LEDs”) has grown tremendously in recent years and the requirement for homogeneity is an important one. Die sorting has become essential especially for semiconductor dice used in applications like displays and lightings. Dice are sorted according to their electrical properties, light intensity, and frequency. Probing to conduct electrical tests on the dice is done either before or after the wafer is singulated or cut into individual dice.
The wafer is usually mounted onto Mylar film and singulation is done on the film. If a wafer is probed before it is singulated, a map file containing the relative logical position and grade of each die will be generated. A mapping sorter will then sort the wafer into different grades of dice according to the map after the wafer is singulated into individual dice. After aligning a die on the physical wafer with the corresponding die on the wafer map, the mapping sorter can start the sorting process. Generally, the position of the next die to be picked will be calculated from its relative wafer map position to the present die and the learnt die pitch of the wafer.
U.S. Pat. No. 6,021,380 entitled “Automatic Semiconductor Wafer Sorter/Prober with Extended Optical Inspection” discloses an automated semiconductor wafer sorter/prober for visually inspecting and sorting semiconductor wafers. Its approach is to obtain a wafer image before probing so that the probe will not be probing at places with no dice, partial dice, or visually defective dice. This is to increase the probing efficiency and to prevent the probes from getting damaged. It also mentions that the system can be used for inspection after the wafer has been singulated and the resulting wafer map can be used for pick-and-place assembly operations. However, it fails to take into account factors such as misalignment of dice or damage to the wafer after singulation of the wafer by a cutting tool. Since it does not record the relative actual physical locations of the dice after singulation of the wafer, it has to pick up dice by relying on the learnt pitch throughout the die pick-up process.
Moving from one die to the next relying on the learnt pitch poses no problems under normal circumstances when the dice are arranged in a regular array. However, this method is unreliable after singulation of the dice. If the next die is far away (for example, dice are missing in between) or the expansion of the Mylar film is uneven, the calculated location of the next die could fall on a wrong physical die (it could be a die in the next row or the next column). If just one die is missed or incorrectly detected, the rest of the dice could be sorted erroneously. The same thing could happen if part of the wafer is damaged after singulation and there are no alignable dice in a certain area.